There have been attempts in the past to superimpose data on a video signal. The most common approach is to insert data during the vertical blanking interval, such as in the generation of closed captioning signals. Another approach has been to place the data on the visible portion of the video signal. One advantage of the latter approach is that it may be possible to detect the data optically as well as electrically.
An example of the optical-detection approach is disclosed in Broughton et al. U.S. Pat. No. 4,807,031. The basic technique disclosed in this patent is to represent data by raising and lowering the luminance of successive horizontal lines within some designated viewing area. Because the average luminance of the two adjacent lines remains the same, the effect is not perceptible to the eye, but sensing of the alternate raising and lowering of the luminance by an appropriate receiver allows the data to be detected. As described in the Broughton et al. patent, the technique is equivalent to superimposing on the video signal a subcarrier frequency of 7.867 kHz, which can be detected by appropriate filtering. Broughton et al. also teach how to determine which fields should have data superimposed on them. For example, fields that are too white or too black are not appropriate for the insertion of data.
It is a general object of our invention to insert data on the visible portion of a video signal by changing the luminance of paired lines in opposite directions, but to do so in a manner that allows for a much higher data rate and the detection of the data in a much more accurate manner than has been possible in the prior art.
As used herein, the term video signal applies to any representation of the standard NTSC, PAL or SECAM signals in common use for video transmission including the analog form, directly digitized numerical representations, CCIR 601/656 standards-based digital representations, computer representations such as RGB or YUV, or other digital representations that are simply numerically converted from the directly digitized representation of standard video. (Encoding and decoding from any digitized form is contemplated as long as it can be determined how the signal was digitized and that information is not lost after digitization.)